Generally, if deterioration of an insulation material that insulates a conductor portion proceeds in the inside of an electric machine, more particularly, a high voltage electric machine, partial discharge generated from the deteriorated portion tends to be increased.
If the partial discharge in the inside of the high voltage electric machine is neglected, dielectric breakdown is likely to occur and it brings about the possibility of a serious accident. Consequently, the partial discharge is detected and the degree of the deterioration of the insulation material needs to be grasped so as not to develop to the serious accident. Therefore, it is effective to place a partial discharge measurement device that measures the partial discharge in the inside of the high voltage electric machine.
When a turbine generator is taken as an example, partial discharge measurement that is performed to grasp the degree of the deterioration of the insulation of the high voltage electric machine is generally classified into offline measurement that is performed during a stop of operation and online measurement that is performed during operation. In the offline measurement, a voltage around the rated voltage is applied to each phase of an armature during the stop of the operation of the turbine generator by using a high voltage power supply and partial discharge generated at that time is measured. Furthermore, in the online measurement, since the rated voltage is generated in the armature during the operation of the turbine generator, the partial discharge generated at that time is continuously measured for a long period.
If the partial discharge is generated, a partial discharge signal having wide frequency components ranging from several kHz to several GHz is generated as a pulse-like signal that propagates in a winding conductor or as an electromagnetic wave that propagates in space; and therefore, a certain frequency band is selected from the partial discharge signal and measurement is performed by using an appropriate sensor.
As for a method of measuring the partial discharge in the inside of the high voltage electric machine, the method can be performed by detecting, for example, current, electromagnetic wave, sound, vibration, and the like by means of a sensor. A method of measuring the electromagnetic wave generated accompanying the partial discharge is expected from detection sensitivity, the goodness of S/N ratio, the width of detection range, and the like. More particularly, a signal ranging from several kHz to several GHz is included in the electromagnetic wave generated accompanying the partial discharge and there has been investigated a method that measures an electromagnetic wave of an ultra high frequency (UHF) band from 300 MHz to 3 GHz.
Here, it is characterized in that, in propagation, a low frequency side signal (from several kHz to several tens MHz) is relatively low in signal attenuation, but is susceptible to noise; and a high frequency side signal (from several hundreds MHz to several GHz) is relatively high in signal attenuation, but is insusceptible to noise. Thus, in the offline measurement which is relatively low in noise level, the low frequency side signal is detected so as to be able to detect even a partial discharge generated in the distance. Furthermore, in the online measurement which is relatively high in noise level, the high frequency side signal insusceptible to noise is detected; and as for large signal attenuation, it is general to place a sensor at a position near a partial discharge source and to manage by applying a sensor with high receiving sensitivity.
Consequently, as a device that detects the partial discharge by using the electromagnetic wave generated accompanying the partial discharge such as this, for example, a partial discharge detection device shown in Patent Document 1 discloses that a waveguide antenna that receives an electromagnetic wave leaked from an insulating spacer is attached to the outer peripheral surface of the insulating spacer, and the waveguide antenna is composed of a waveguide portion and a coaxial cable connector placed on the waveguide portion; the waveguide portion is provided with a metallic short-circuit plate at one end and is formed with an opening portion that covers a part of the outer peripheral surface of the insulating spacer at the other end; and thus, even when a detector of the partial discharge is not preliminarily attached to the inside of a gas-insulated device, the partial discharge can be detected by detecting the electromagnetic wave leaked from the gas-insulated device.